In the case of apparatuses for detecting impurity particles in fluids and in particular microscopic impurity particles, it has been found that the measured result can be falsified by dust particles from the ambient air travelling in the optical path, because they can give rise to an error signal, although the bottle or fluid being investigated is itself fault-free. Such an apparatus is fundamentally known from DE-OS 37 03 306, whose disclosure is made part of the subject matter of the present application and to which reference is made.